The present invention relates to a glow plug temperature control apparatus for a Diesel engine in which a glow plug is employed to preheat a combustion chamber to facilitate starting the engine.
In the prior art a glow plug is energized with electrical power and thereby raised to a high temperature for a predetermined period of time. Then, an attempt is made to start the engine. If the engine does not start the first time, the glow plug is energized again for the same period of time and another attempt made to start the engine. This procedure is repeated until the engine starts.
Improved glow plugs have been developed which heat up very quickly when electrically energized and tend to overheat and fuse the heating elements thereof if energized for too long. Energizing such a glow plug for a correct length of time from a cold condition will heat it to the desired temperature. However, after the glow plug is de-energized the temperature thereof does not fall instantaneously. Instead, it falls exponentially at a rate which may be determined experimentally.
For this reason, if the glow plug is initially heated and an unsuccessful attempt is made to start the engine, the glow plug will again be energized for the predetermined length of time and another attempt made to start the engine. However, the second heating of the glow plug will begin before the glow plug has had time to cool to ambient temperature, and the temperature of the glow plug will still be quite high. Since the second heating of the glow plug begins while the glow plug is still at an elevated temperature, it will result in the glow plug being heated to an excessive temperature. Several unsuccessful attempts to start the engine and corresponding heating operations of the glow plug will cause the temperature of the glow plug to rise so high that the heating elements thereof will fuse and result in destruction of the glow plug.